1. Field of the Invention
This invention finds application in certain measurement systems which determine the heading of a borehole of a well. For example, the invention relates to measuring-while-drilling systems (MWD) which are designed to determine the position and heading of a tandemly connected sub near the drill bit of a drill string assembly in an oil or gas well borehole. The invention also finds application with wireline apparatus in which one or more down-hole instruments are designed to determine the position and heading of such instrument(s) during logging of an open hole borehole. In particular, the invention relates to the determination of the heading of the well from gyroscopic data regarding the earth's rotation and from accelerometer data regarding the earth's gravitational field. Still more particularly, the invention relates to an apparatus and method for compensating gyroscopic data for movement of a down-hole measurement instrument while a heading determination is being made.
2. Description of the Prior Art
Prior art measuring-while-drilling equipment has included magnetometers and accelerometers disposed on each of three orthogonal axes of a measurement sub of a drill string assembly. Such measurement sub has typically been part of a special drill collar placed a relatively short distance above a drilling bit. The drilling bit bores the earth formation as the drill string is turned by a rotary table of a drilling rig at the surface.
At periodic intervals, the drill string is stopped from turning so that the measurement sub in the well boremay generate magnetometer data regarding the earth's magnetic field and accelerometer data regarding the earth's gravitational field with respect to the orthogonal axes of the measurement sub. The h vector from the magnetometer data and the g vector from the accelerometer data are then used to determine the heading of the well.
Such prior art method suffers from the fact that the earth's magnetic field varies with time and is affected by structures containing iron or magnetic ores in the vicinity of the measurement sub. Such variation leads to errors and uncertainty in the determination of the well heading.
Such variation in the heading determination of the measurement sub of a MWD assembly, or a similar wireline instrument, can theoretically be eliminated by adding gyroscopes to each of the orthogonal axes of the measurement sub. In theory, the heading of the measurement sub can then be determined from accelerometer data from each of such axes and gyroscopic data from each of such axes. The accelerometer data is responsive to the gravitational field of the earth, while the gyroscopic data is responsive to the rotational velocity of the earth with respect to inertial space.
Movement of the measurement sub (in the case of an MWD application) while accelerometer and gyroscopic data is being taken can introduce an error into the determination of the earth's rotational velocity vector. Such movement may be caused by the "twist" or torque on the drill string after it is stopped from rotation and it is suspended from slips in the rig rotary table. Such twisting motion may occur on land rigs or on floating drilling rigs. Motion may also be produced while drilling has been suspended for a heading determination in a floating drilling rig where the heave of the sea causes the drill string to rise and fall in the borehole. Rotation of such drill string may be caused due to wave induced reciprocation of the measurement sub along a curved borehole. Analogous errors may occur in the case of a wireline instrument.